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What causes blood clots?

What causes blood clots?

Navin Khosla NowPatientGreen tick
Updated on 13 May 2024

Blood clots are a common medical condition that can have serious consequences if left untreated. Blood clots form in the body’s arteries and veins, obstructing the normal flow of blood and potentially leading to life-threatening complications. Understanding the various causes and risk factors associated with blood clots is important for their prevention, early detection and effective management.

What are blood clots?

Blood clots are the body’s natural defence mechanism against excessive bleeding. The body triggers a complex cascade of events called coagulation that results in the formation of a clot when a blood vessel is damaged. This clot (consisting of platelets, proteins, and cells) helps to plug the injured area and stop the bleeding. However, in some cases, blood clots form without an obvious injury or may not dissolve as they should, leading to potentially dangerous situations.

What causes blood clots

Some of the primary causes of blood clot formation include:


Atherosclerosis is a condition identified by the build-up of plaque in the arteries, leading to the formation of blood clots. As the plaque builds up, it narrows the arterial passage and the body responds by forming a clot to try and restore blood flow. These clots can then break off and move to other parts of the body, potentially causing a heart attack or stroke.

Atrial fibrillation

Atrial fibrillation is a type of irregular heartbeat that can contribute to the development of blood clots. During atrial fibrillation, the upper chambers of the heart (the atria) do not contract effectively, allowing blood to pool and form clots that can then travel to other parts of the body.

Genetic factors

An individual’s risk of developing blood clots is higher with certain genetic disorders, such as Factor V Leiden thrombophilia, prothrombin gene mutation, and protein C or S deficiency. These genetic conditions can cause the blood to clot more easily or make it more difficult for the body to break down existing clots.

Medical conditions

Cancer, diabetes, high blood pressure, high cholesterol, and chronic inflammatory diseases like rheumatoid arthritis and lupus can increase the risk of blood clot formation.

Lifestyle factors

Obesity, smoking, and inactivity (such as during long flights or hospital stays) can all contribute to an increased risk of clot formation.

Pregnancy and postpartum period

Pregnancy and the postpartum period are also times when women are at a higher risk of developing blood clots. The hormonal changes and increased pressure on veins during pregnancy can make it more likely for clots to form, particularly in the legs and pelvis.


Certain medications, such as birth control pills, hormone replacement therapy and some cancer treatments, can also increase the risk of blood clot formation.

Risk factors for blood clots

In addition to the various causes, several risk factors can make an individual more susceptible to developing blood clots. These include:

  • Age: The risk of blood clots increases with age, particularly for individuals over 65
  • Obesity: Excess weight can slow the flow of blood in the veins, increasing the risk of clot formation
  • Immobility: Prolonged periods of inactivity, such as during hospitalization or long-distance travel can lead to the development of blood clots, especially in the legs
  • Smoking: Smoking can damage the lining of the blood vessels, making it more likely for clots to form
  • Family history: Individuals with a family history of blood clots may have a genetic predisposition to the condition
  • Recent surgery or Injury: Trauma to the body, such as from surgery or injury can trigger the body’s clotting mechanisms and increase the risk of clot formation
  • Pregnancy and postpartum Period: As mentioned earlier, the hormonal changes and increased pressure on the veins during pregnancy and the postpartum period can heighten the risk of blood clots
  • Cancer: Certain types of cancer, as well as some cancer treatments, can increase the risk of blood clot formation
  • Chronic inflammatory conditions: Diseases like rheumatoid arthritis and lupus can contribute to the development of blood clots
  • Dehydration: Inadequate fluid intake can thicken the blood, making it more prone to clotting

Symptoms of blood clots

The symptoms of blood clots can vary depending on the location of the clot and the severity of the condition. Some common symptoms include:

  • Deep vein thrombosis (DVT): Sudden or gradual pain, swelling, tenderness, and warmth in the affected limb (usually the leg)
  • Pulmonary embolism (PE): Shortness of breath, chest pain, rapid breathing, and increased heart rate
  • Cerebral venous sinus thrombosis (CVST): Trouble speaking, vision problems, seizures, weakness on one side of the body, and sudden severe headache
  • Heart attack: Chest pain, sweating, shortness of breath, and pain in the left arm
  • Stroke: Weakness of the face, arms, or legs, difficulty speaking, and sudden and severe headache

It is crucial to seek immediate medical attention if any of these symptoms are present, as blood clots can be life-threatening if left untreated.

Diagnosis and testing

To diagnose blood clots, healthcare providers may use a combination of physical examinations, medical history, and various diagnostic tests, including:

  • Blood tests: These may include a D-dimer test, which can detect the presence of clot-dissolving proteins in the blood
  • Imaging tests: Techniques such as ultrasound, CT scans, and MRI can help visualize the location and extent of the clot
  • Venography: This X-ray procedure involves injecting a contrast dye into the veins to allow for a detailed view of the blood flow
  • Angiography: Similar to venography, this X-ray technique is used to examine the arteries and detect any blockages or clots

By using a combination of these diagnostic tools, healthcare providers can accurately identify the presence and location of a blood clot, enabling them to develop an appropriate treatment plan.

Treatment and management of blood clots

Treatment for blood clots depends on the location and severity of the clot, as well as the underlying cause. Common treatment approaches include:

  • Anticoagulants (blood thinners): These medications, such as heparin, warfarin, and newer oral anticoagulants, help prevent the formation of new clots and the growth of existing ones
  • Thrombolytics: These clot-dissolving drugs, also known as “clot busters,” are used to break down existing blood clots, particularly in severe cases
  • Catheter-directed thrombolysis: This procedure involves the surgical insertion of a catheter to deliver clot-dissolving medication directly to the site of the clot
  • Thrombectomy: In some cases, the surgical removal of a blood clot may be necessary, especially for large or life-threatening clots

Preventing blood clots

While some risk factors for blood clots, such as age and genetic predisposition cannot be controlled, there are several steps including lifestyle changes individuals can take to help reduce their risk of developing blood clots:

  • Staying active: Regular physical activity, even something as simple as taking regular breaks to walk around during long periods of sitting, can help improve blood flow and reduce the risk of clot formation
  • Maintaining a healthy weight: Excess weight can slow the flow of blood in the veins, increasing the risk of clot formation. Achieving and maintaining a healthy body weight can help mitigate this risk
  • Quitting smoking: Smoking can damage the lining of the blood vessels, making them more susceptible to clot formation. Quitting smoking can significantly reduce the risk of blood clots
  • Staying hydrated: Dehydration can thicken the blood, making it more prone to clotting. Drinking plenty of water and other fluids can help prevent this
  • Alternatives to estrogen-based birth control: Estrogen-containing birth control pills and hormone replacement therapies can increase the risk of blood clots. Discussing alternative options with a healthcare provider can help reduce this risk
  • Seeking medical attention for prolonged immobility: If an individual is required to be immobile for an extended period, such as during a hospital stay or long-distance travel, they should consult with a healthcare provider about preventive measures, such as the use of compression stockings or blood thinners
  • Underlying conditions: Individuals with conditions that increase the risk of blood clots, such as cancer, diabetes, or inflammatory diseases, should work closely with their healthcare providers to manage these conditions and take appropriate preventive measures

By understanding the causes, risk factors, and preventive strategies for blood clots, individuals can take proactive steps to protect their health and reduce the likelihood of developing this potentially life-threatening condition.


Understanding the various causes, risk factors and preventive methods is important for individuals to take an active role in their healthcare and reduce their chances of developing blood clots. By working closely with healthcare providers, maintaining a healthy lifestyle, and being vigilant about prevailing symptoms, individuals can take proactive steps to safeguard their cardiovascular health and reduce the risks of developing blood clots.


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NowPatient has taken all reasonable steps to ensure that all material is factually accurate, complete, and current. However, the knowledge and experience of a qualified healthcare professional should always be sought after instead of using the information in this page. Before taking any drug, you should always speak to your doctor or another qualified healthcare provider.

The information provided here about medications is subject to change and is not meant to include all uses, precautions, warnings, directions, drug interactions, allergic reactions, or negative effects. The absence of warnings or other information for a particular medication does not imply that the medication or medication combination is appropriate for all patients or for all possible purposes.

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